Technical Field
This invention is generally related to catalysts and catalytic forms and formulations for use in natural gas processes, such as the oxidative coupling of methane.
Description of the Related Art
Catalysis is the process in which the rate of a chemical reaction is either increased or decreased by means of a catalyst. Positive catalysts lower the rate-limiting free energy change to the transition state, and thus increase the speed of a chemical reaction at a given temperature. Negative catalysts have the opposite effect. Catalysts are generally characterized as either heterogeneous or homogeneous. Heterogeneous catalysts exist in a different phase than the reactants (e.g., a solid metal catalyst and gas phase reactants), and the catalytic reaction generally occurs on the surface of the heterogeneous catalyst. Thus, for the catalytic reaction to occur, the reactants must diffuse to and/or adsorb onto the catalyst surface. This transport and adsorption of reactants is often the rate limiting step in a heterogeneous catalysis reaction. Heterogeneous catalysts are also generally easily separable from the reaction mixture by common techniques such as filtration or distillation.
One heterogeneous catalytic reaction with commercial potential is the oxidative coupling of methane (“OCM”) to ethylene: 2CH4+O2→C2H4+2H2O. See, e.g., Zhang, Q., Journal of Natural Gas Chem., 12:81, 2003; Olah, G. “Hydrocarbon Chemistry”, Ed. 2, John Wiley & Sons (2003). This reaction is exothermic (ΔH=−67 kcals/mole) and has typically been shown to occur at very high temperatures (>700° C.). Although the detailed reaction mechanism is not fully characterized, experimental evidence suggests that free radical chemistry is involved. (Lunsford, J. Chem. Soc., Chem. Comm., 1991; H. Lunsford, Angew. Chem., Int. Ed. Engl., 34:970, 1995). In the reaction, methane (CH4) is activated on the catalyst surface, forming methyl radicals which then couple in the gas phase to form ethane (C2H6), followed by dehydrogenation to ethylene (C2H4). To date, the OCM reaction has not been commercialized, due in large part to the lack of effective catalysts and catalytic forms.
Another catalytic reaction with commercial potential is the oxidative dehydrydrogenation (ODH) of ethane to ethylene. Oxidative dehydrogenation of ethane to ethylene has been proposed to replace thermal cracking of ethane. The lower temperature operation and exothermic nature of ODH has the potential to significantly reduce the external heat input required for thermal cracking and lessen the coke formation. However, over oxidation of ethylene can reduce the ethylene selectivity, and better catalysts and processes are needed before the full potential of this reaction can be realized.
Many heterogeneous catalysts are employed in combination with a binder, carrier, diluent, support material and/or are provided in specific shapes or sizes. The use of these materials provides certain advantages. For example, supports provide a surface on which the catalyst is spread to increase the effective surface area of the catalyst and reduce the catalyst load required. The support or diluent may also interact synergistically with the catalyst to enhance the catalytic properties of the catalyst. Further, catalytic supports may be tailored to specific reactions and/or reactor types in order to optimize the flow (e.g., reduce back pressure) of gaseous reactants.
While some progress has been made, there remains a need in the art for improved catalysts, catalyst forms and formulations and catalytic processes for use in catalytic reactions, such as OCM and ODH. The present invention fulfills these needs and provides further related advantages.